What’s the output of systemctl | grep backlight? eg here’s mine:
# systemctl | grep backlight
sys-devices-pci0000:00-0000:00:02.0-drm-card0-card0\x2deDP\x2d1-intel_backlight.device loaded active plugged /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight
sys-devices-platform-dell\x2dlaptop-leds-dell::kbd_backlight.device loaded active plugged /sys/devices/platform/dell-laptop/leds/dell::kbd_backlight
systemd-backlight@backlight:intel_backlight.service loaded active exited Load/Save Screen Backlight Brightness of backlight:intel_backlight
systemd-backlight@leds:dell::kbd_backlight.service loaded active exited Load/Save Screen Backlight Brightness of leds:dell::kbd_backlight
system-systemd\x2dbacklight.slice loaded active active system-systemd\x2dbacklight.slice